Influence of historical manmade alterations on soil-forming processes in a former imperial estate (Shirogane-goryouchi), the Institute for Nature Study: Development of a soil evaluation technique and importance of inventory construction for urban green areas

The influence of historical land use on soil-forming processes was assessed in a former imperial estate (Shirogane-goryouchi), the Institute for Nature Study, as one of the urban green areas where many records on land use and vegetation history have been maintained. The area of study was divided into several subsections and categorized based on land use history. Results from a soil survey in each categorized area showed that Andosols were dominant across all areas, and that past construction activities, creation of artifacts, and soil cutting and banking resulted in a disruption of horizon sequences and shifts in soil components. Soil was characterized using chemical analyses of oxalate-extractable Al (Alo) and Fe (Feo), the melanic index (MI) of soil organic matter (SOM), and the carbon stable isotope ratio (δ ¹³C) of soil. These parameters helped researchers assess the influence of land use on soil and vegetation over time. After land use ceased, newly accumulated organic horizons, the development of soil structures, and horizon differentiation due to forest regeneration were apparent. δ ¹³C of soil suggested that SOM in surface horizon (A, A1) was primarily derived from present-day forest vegetation (C3 plants) with MI values >1.7. The origins of SOM in subsurface horizon (A2, AB) were mixed with present-day C3 plants and past grass vegetation from C4 plants. MI values were lower in subsurface horizons due to a higher degree of humification as compared to surface horizon values. The absence of fresh organic matter supply after deforestation and lack of soil material mixing between surface and subsurface horizons after land use changed affected the state of soil organic matter in the subsurface horizon. Alo leached from surface to deeper horizons via large inputs of organic material over the long term (approximately 400 years), even on sections of earthworks. Even though the soil in this urban green area had a complex land use history, it was still possible to evaluate the soils in terms of pedogenesis with a multidisciplinary approach that explored land use and vegetation history, and used soil surveys and soil chemical characteristics to answer our questions.